SummaryWater-soluble organic dyestuffs used for dyeing the anodic coatings on aluminium cause hardly any damage to the biocenosis of the water into which the effluents of anodizing plants are released, as ecological laboratory investigations and years of practical experience have shown. However, the extremely high tinctorial strength of such dyes, even after being heavily diluted (less than 0·1 ppm) cause conspicuous and unsightly discolouration of the water into which the effluent is released. Since in mechanical-biological water purification systems these dyestuffs can only be eliminated in insignificant amounts, concentrated dye solutions must be decolourized before being drained. Coloured running rinse water, however, due to its slight discolouration, normally requires no special treatment.A number of different processes are available for eliminating dyestuffs by chemical decomposition or by physical separation, such as adsorption by activated carbon, precipitation, reverse osmosis, evaporation and incineration, catalyzed, radioactivated or electrochemical wet oxidation under aeration, chemical oxidation (e g with ozone), or chemical reduction (e g with hydrosulphite).Adsorption with activated carbon represents an especially economical and flexible solution to the problem of decolonization. This process can be carried out both in a stationary manner by the suspension principle as well as by the percolation principle. Moreover, there are no problems relating to secondary pollution of the effluent or with disposing of the solid waste.